The present invention is related to a technique of bonding or adhering polyurethane to a condensation type silicone rubber, and in particular to a technique of surface modification of a condensation type silicone rubber to facilitate the bonding between the condensation type silicone rubber and polyurethane.
The unusual heat, electrical weather, and low-temperature resistance properties of silicones, i.e., room temperature-vulcanizable (RTV) silicone rubber, have manifested themselves in a wide variety of applications. They are used as engineering materials for manufacturing electrical apparatus and machines. Strong adhesion of silicones to the target matrix is a requirement for long product life. The inherently hydrophobic nature of silicones, coupled with their ability to segregate to the surface, facilitates their use as a surface modifier for other materials. This property has been exploited in the preparation of silicone-modified polyesters; the synthesis of polydimethylsiloxane (PDMS) and polystyrene copolymers; polyimidesiloxane segmented copolymers triblock siloxane copolymers used as surface modifying additives; blood-compatible polyurethane (PU)-polysiloxane graft copolymers [Y. Ezuka, T. Ono, and K. Imai, J. Colloid Interface Sci., 136, 408 (1990)]; and the surface treatment of fumed silica for use as antifoaming compounds. Small amounts of the copolymers (ca. 1xcx9c4 by weight) could be added to different polymers to alter their surface properties. The air-polymer surface of the resulting polymer system is usually dominated by the low surface-energy siloxane, preventing adhesion to polar materials. This phenomenon of surface segregation of a component with a low surface energy in multicomponent polymeric materials attracts attention to the analysis of polymer surfaces and the modifications of their properties [G. H. Fredrickson, Macromolecules, 20, 2535 (1987); T. P. Russell, G. Coulon, V. R. Deline, and D. C. Miller, Macromolecules, 22, 4600 (1989); K. R. Shull, Macromolecules, 25, 2122 (1992)].
Surface modification of polymers by chemical, photochemical, corona, plasma, and high-energy radiation treatments are currently used to increase the polarity of polymer surfaces, thus enhancing their adhesion to immiscible polymers [C. M. Chan, Polymer Surface Modification and Characterization, Hanser Publishers, Munich, 1993; W. H. Waddall, L. R. Evans, J. G. Gillick, and D. Shuttleworth, Rubber Chem. Technol., 65, 687 (1992)]. An alternative means of modifying polymer surface is by blending with diblock copolymers, where one block interacts favorably with the base matrix and the other block with the target matrix [T. P. Russell, V. R. Deline, V. S. Wakharkar, and G. Coulon, MRS Bull., October, 33 (1989)]. The polymeric surface can form ordered, oriented layers at the surface during subsequent curing. The part of such copolymers with a low critical surface tension is readily miscible with the silicone base matrix, whereas the other part interacts favorably with the more polar matrix. Both blocks can anchor the copolymer into the respective substrate, which ensures permanency of the surface modification and increases adhesion between two immiscible polymers.
A condensation type silicone rubber contains residual SiOH functional groups on its surface after being cured; however, the residual SiOH functional groups do not form reliable chemical linkages with xe2x80x94NCO groups of a polyurethane when them are brought into contact with each other. Further, the low surface energy of the cured silicone rubber causes the polyurethane have insufficient wetability to adhere to the surface thereof.
A primary objective of the present invention is to provide a process for preparing a laminated composite of a condensation type silicone rubber and polyurethane.
Another objective of the present invention is to provide a process for preparing a condensation type silicone rubber having an increased surface energy by induce surface reconstruction.
Still another objective of the present invention is to provide a modifier for increasing a surface energy of a condensation type silicone rubber.
In order to accomplish the objectives of the present invention, a process for preparing a laminated composite of a condensation type silicone rubber and a polyurethane carried out In accordance with the present invention comprises the following the steps:
(a) mixing a modifier, a silicone gel of condensation type and a curing agent for said silicone gel to form a mixture, said modifier having the following formula (I): 
xe2x80x83wherein m=25xcx9c50; R1, R2, R3, R4, R11 and R12 independently are xe2x80x94CH3 or xe2x80x94CH2CH3; Rxe2x80x2 is R or OR, wherein R is a polymer backbone having a molecular weight of 1000xcx9c20000;
(b) introducing said mixture from step (a) into a cavity of a mold, in which a surface of the cavity is formed by a material having a critical surface tension greater than that of a polymer having a repeating unit of said R;
(c) curing said mixture in said cavity to form a shaped article;
(d) removing said shaped article from said mold;
(e) placing a polyurethane precursor on a surface of said shaped article which contacts said surface of said cavity; and
(f) curing said polyurethane precursor to form an cured polyurethane bonding to said surface of said shaped article, so that a laminated composite is formed.
Preferably, the process of the present invention further comprises (dxe2x80x2) removing volatile alcohol from said shaped article formed during said curing in step (c) by evaporation prior to step (e).
In step (e), a suitable polyurethane precursor is a liquid mixture comprising a polyol, and a polyisocyanate, and optional a catalyst, wherein OH group:NCO group=0.7:1 to 0.9:1 (molar), preferably 0.8:1.
Alternatively, the polyurethane precursor is a polyurethane elastomer having a tensile strength of 20xcx9c40 kg/cm2 and an elongation ratio at break of 200xcx9c400%.
The curing in step (f) of the process of the present invention is preferably carried out at a temperature of 60xc2x0 C. to 80xc2x0 C. for a period longer than 120 hours.
The present invention also discloses a process for preparing a condensation type silicone rubber having an enhanced critical surface tension, which comprises steps (a) to (c) defined as above.
Preferably, R in the formula (I) is 
or a combination of each other, wherein a is an integer of 30xcx9c100, preferably a is an integer of 40xcx9c60.
A suitable curing agent for said silicone gel in step (a) of the process of the present invention Includes (but not limited thereto) compounds having a formula of (R20)nSi(OR21)4xe2x88x92n, wherein n is an integer of 0, 1 or 2; R20 is xe2x80x94CH3 or xe2x80x94CH2CH3; and R21 is xe2x80x94CH3, xe2x80x94CH2CH3, or xe2x80x94CH2CH2CH3.
Preferably, said silicone gel of condensation type used in step (a) is a silicone having the following formula (II): 
wherein k is a value so that the silicone (II) has a molecular weight of 1000xcx9c10000; and R5, R6, R7, R8, R9 and R10 independently are xe2x80x94CH3or xe2x80x94CH2CH3.
A suitable amount of said modifier used in step (a) ranges from 0.5 to 10.0 phr (per hundred parts by weight of said silicone gel of condensation type), and preferably from 1.0 to 3.0 phr.
Preferably, said curing in step (c) is carried out at a temperature from room temperature to 50xc2x0 C. for a period of 12 to 24 hours.